Known hemodialysis machines have a control device which makes it possible to preselect a certain ultrafiltration rate. This device ensures that a predetermined quantity of ultrafiltrate is removed from the patient during the hemodialysis treatment, regardless of the viscosity of the blood to be treated and the properties of the dialyzer.
Since a defect in the device for monitoring the ultrafiltration can endanger the patient, safety standards require the presence of a safety system as a safeguard against ultrafiltration that would be hazardous for the patient. The most commonly used safety system monitors the transmembrane pressure for this purpose.
However, one result of the development of dialyzers with high permeability membranes, known as high-flux dialyzers, has been that an excessively high or low ultrafiltration rate which could endanger the patient cannot be detected with sufficient reliability by monitoring the transmembrane pressure.
German Patent Application No. 42 39 937 describes a method of determining the functionality of a partial device of a hemodialysis machine, where the dialyzer is separated from the dialysis fluid circuit for short periods of time at periodic intervals during dialysis, and the pressure profile in the dialysis fluid is determined by means of a pressure-holding test for deviation from the stable state. This method has proven successful in practice. However, a disadvantage of this is that the known method where the dialyzer is separated from the dialysis fluid circuit does not include the dialyzer in the testing. Thus defective O rings on the dialyzer couplings, which are a common cause of leakage, cannot be detected.